For slitting or shearing off scrap from a piece of sheet metal held on a work table, a type of shear having dual tractive cutters, as shown in U.S. Pat. No. 2,946,251 has come into common use. In this type of shear, upper and lower rotary cutters, whose edges are adjacent to each other in cutting relationship along a vertical plane of cut, are mounted on lateral shafts through forward parts of upper and lower frame portions, joined together by an upward slanting bridge part aft of the cutters. The upper frame portion lies inward of the line of cut and passes above the horizontal work surface on which the sheet to be cut is laid and clamped. At the intersection of the rotary cutters, the lower frame portion is entirely beneath the work surface level. Aft of the cutters its upper edge slants upward and aft, to overlap and join the aft lower part of the upper frame portion. This upward slanting joining part of the frame is referred to as its bridging portion.
In such prior art cutter, an electric motor is mounted onto the outer side of the lower frame portion. This motor provides power to both cutters; necessarily its diameter is large and it is mounted low. After its power is transmitted first to a gear on the lower cutter shaft, a sprocket on this shaft drives a chain which extends slantingly upward to a sprocket near the aft end of the sloping bridge juncture. The power is then transmitted to the inner side of the upper frame, and, its direction of rotation being reversed, is carried by a chain and sprocket to drive the upper rotary cutter. This is driven at the same speed as the lower rotary cutter but in an opposite sense.
One function of the upward sloping edge of the bridging part is to deflect the scrap being cut from the sheet slantingly upwards over the bridging part. To accommodate power transmission mechanism beneath the deflected scrap requires a fairly large overlap of the outer and inner frame parts, and hence a substantial upward slope angle. Especially when thicker sheets are to be cut, the need to deflect the scrap upwardly adds to the power requirement of the shear.